Dementia is a clinical syndrome characterized by deficits in multiple areas of cognition that cannot be explained by normal aging, a noticeable decline in function, and an absence of delirium. In addition, neuropsychiatric symptoms and focal neurological findings are usually present. Dementia is further classified based on etiology. Alzheimer's disease (AD) is the most common cause of dementia, followed by mixed AD and vascular dementia, vascular dementia, Lewy body dementia (DLB), and fronto-temporal dementia.
The incidence of Alzheimer's disease is expected to increase through the year 2050 with an estimated prevalence of 11 to 16 million cases. Currently, two classes of medications are FDA approved for managing symptoms of AD—acetylcholinesterase inhibitors (AChEIs) and an N-methyl-D-aspartase (NMDA) receptor antagonist. AChEIs are commonly used as initial treatment on diagnosis. The AChEIs—donepezil, rivastigmine, galantamine, and tacrine—are indicated for mild-to-moderate AD; only donepezil is approved for the severe stage.
Despite the available therapies, there are no treatments to cure AD or to prevent or stop disease progression. Acetylcholinesterase inhibitors do not help everyone who has Alzheimer's disease and in fact are not efficacious in many patients. Considering that AChEIs and memantine have only a modest symptomatic effect, and cannot prevent AD decline and slow disease progression, there is a high unmet need for more effective symptomatic treatments and for a disease modifying/slowing therapies.
The use of selective 5-HT6 receptor antagonists to treat cognitive dysfunction has been suggested and is based on several lines of reasoning. For example, selective 5-HT6 receptor antagonists have been shown to modulate cholinergic and glutamatergic neuronal function. The activity of selective 5-HT6 receptor antagonists has been demonstrated in animal models of cognitive function. Since the disclosure of the first selective 5-HT6 receptor antagonists, there have been several reports on the activity of these selective compounds in in-vivo models of cognitive function. N-[2-(6-fluoro-1H-indol-3-yl)ethyl]-3-(2,2,3,3-tetrafluoropropoxy)benzylamine (herein referred to as “Compound I”) is a potent and selective 5-HT6 receptor antagonist which has been in clinical development for treating cognition impairment associated with schizophrenia and as a treatment for AD.

In November 2008, a multi-centre, randomised, double-blind, fixed-dose study (120 mg/day BID) was initiated to explore the efficacy and safety of Compound I as adjunctive treatment to risperidone in patients with schizophrenia. Overall improvement in schizophrenia symptoms was assessed by using the Positive and Negative Syndrome Scale (PANSS) total score. Compound I did not offer any treatment advantage over placebo as measured by the PANSS total score. In 2010, it was announced that there did not appear to be any treatment advantage over placebo in improving patients' overall neurocognitive performance as assessed using the BACS composite Z-score and the PANSS cognitive subscale scores.
In 2012, it was reported that a randomized, double blind, placebo controlled trial conducted in Europe, Canada and Australia met its primary endpoint in the treatment of AD. Data demonstrated that Compound I plus 10 mg/day of donepezil significantly improved cognitive function in 278 patients with Alzheimer's disease compared to placebo plus donepezil, when measured by Alzheimer's Disease Assessment Scale-cognitive sub-scale (ADAS-cog). Compound I showed positive results in secondary endpoints including measures of global impression and daily living activities compared to donepezil treated patients.
The daily dose of 90 mg of Compound I in the AD study was administered three times daily (3×30 mg) to overcome the relative short half-life observed in subjects in previous clinical studies. An issue for that dose selection was to ensure that the maximum exposure level fell under the maximum exposure limit which had been established from non-clinical toxicology studies. Accordingly, a fixed dose of three times in the study was introduced.
As the 5-HT6 receptor is a novel target predominately localized in the brain, a key problem in the development is to determine the amount of CNS penetration and the correlation with plasma exposure.
With CNS targets, further challenges exist that revolve around whether a drug will pass the blood brain barrier and whether it will reach the target at a suitable concentration and for a sufficient length of receptor occupancy.
Direct brain measures of 5-HT6 receptor occupancy may be valuable to many decision-making processes during the development of centrally acting drugs targeted at 5-HT6 to ensure adequate proof-of-concept testing and to optimize dosing regimens. In humans, tools such as positron emission tomography (PET) with specific radiolabeled ligands have been used to quantitatively assess in-vivo occupancy of a number of neurotransmitter receptors, including those for dopamine, serotonin, and benzodiazepines (Talbot, et al., European Neuropsychopharmacology, 2002, 12, 503-511).
The inventors discovered an effective PET ligand, [11C]-LuPET, which has since been successfully evaluated for human use. The ligand was subsequently used to determine the 5-HT6 receptor occupancy following multiple dose ranges of Compound I. In the assessment for receptor occupancy, human subjects were administered the compound for at least three days at several dosage regimens.
The inventors discovered that high levels of receptor occupancies were observed after multiple dosages of Compound I and that receptor occupancy was maintained 24 hrs post dose. Data generated from a separate Phase I PK study in the elderly and data generated from the above AD study have shown that the elimination half life of Compound I in the elderly population was longer (about 19 hours) compared to young healthy subjects (about 12 hours).
With these convergent discoveries, the inventors have identified improved methods of treating AD by introducing a new and improved dosage regime comprising once daily administration in a novel dosage range. Based on the findings described herein, the dose range contemplated is expected to be efficacious while providing exposure levels below the NOAEL, thus improving the safety ratio. The invention is described in described in greater detail below.